I don't know what to do to fix the issue, if its a driver problem, then please tell me how to download and install the drivers.

NOTE- I an using an usb wifi adapter (TL-WN722N) to connect to internet.
NOTE2-I use mint in dual-boot with windows 10. Their is no issue with windows 10 as well as ubuntu 16.04 which I used previously in the same machine.

I suspect the driver being used is not the correct one, shown below. You should check what the device itself identifies as using lsusb command. Maybe then we can determine the correct driver for it. It may be you have to do similar as in the above link.

I suspect the driver being used is not the correct one, shown below. You should check what the device itself identifies as using lsusb command. Maybe then we can determine the correct driver for it. It may be you have to do similar as in the above link.

The internal wifi card works fine but sometimes, automatically gets switched off until I reboot the pc.

So, this doesn't even seem a wifi problem as much as a power control problem. Have you checked your power settings to see if some of them may be causing this? Just to check it out, make all power off timed settings be many hours, or turned off totally, see if the problem continues.

I saw on Utube that setting your router to only broadcast on one channel (static rather than dynamic) was a fix. I'm trying it now and it seems to have helped thou it's only been a couple days. NOTE: it helps to check on what channels other wifi networks in your area are using so you can select one that is possibly more "isolated"